1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to multi piece golf balls having a core, a mid layer and a cover.
2. Description of the Related Art
Top concern to golf players for golf balls is their flight performances. The golf players particularly place great importance on flight distance attained upon shots with a driver. The golf players also place great importance on flight distance attained upon shots with a long iron and a middle iron.
Golf balls have numerous dimples on the surface thereof. The dimples cause turbulent flow separation through disrupting the air flow around the golf ball during the flight. By causing the turbulent flow separation, a separating point of air from the golf ball shifts backwards leading to the reduction of a drag. The turbulent flow separation promotes the differentia between upper separating point and lower separating point of the golf ball, which results from the backspin, thereby enhancing the lift force that acts upon the golf ball. The dimples are responsible for flight distance.
Golf players also place great importance on spin performances of golf balls. Great back spin rate results in small run. For golf players, golf balls which are liable to be spun backwards are apt to be rendered to stop at a targeted position. Great side spin rate results in easily curved trajectory of the golf ball. For golf players, golf balls which are liable to be spun sidewise are apt to allow their trajectory to curve intentionally. Golf balls that are excellent in spin performances are excellent in control performances. High-level golf players particularly place great importance on control performances upon shots with a short iron.
For golf players, durability of golf balls is also important. Golf balls that are not damaged by repeated hitting have been desired. Further, golf balls that are resistant to generation of cuts and wrinkles on the cover upon full shots with a short iron have been desired.
In light of the flight performances, control performances and durability, a variety of improvement relating multi piece golf balls have been proposed. For example, U.S. Pat. No. 5,823,890 (Japanese Patent No. 3000918 B) proposes a golf ball having a spherical center, a mid layer comprising an ionomer resin which is obtained by compression molding and a cover comprising an ionomer resin which is obtained by compression molding.
Examples of methods of covering a core with a cover include compression molding and injection molding. In injection molding, a core is held in a cavity of a mold. Around the core, a melted cover material is injected. In compression molding, a bowl-shaped half shell comprising a cover material is formed. A core is covered by two half shells. The core and the half shells are compressed and heated in a mold.
In general, a resilience performance of a resin used for a cover is inferior to a resilience performance of rubber used for a center. In light of a resilience performance of a golf ball, a thin cover is preferred. In injection molding, a thickness of a cover depends on the distance between a core and a cavity face. To form a thin cover by injection molding is difficult. Thus, compression molding is adopted to form thin covers. In this compression molding, however, a mid layer material also melts when being compressed and heated. In compression molding, it is likely that the melted mid layer material flows out from a seam of two half shells. When the outflow is generated, appearance of a golf ball is impaired by exposing the mid layer on a seam line of the golf ball. Further, when the golf ball is repeatedly hit, the exposed mid layer easily cracks, which may break the cover. The exposure deteriorates durability of the golf ball.
As mentioned above, golf balls have numerous dimples on the surface thereof. The dimples are recessed and a thickness of the cover immediately below the dimples is less than a thickness of the cover immediately below a land. When golf balls have a small nominal thickness of the cover, a thickness of the cover immediately below the dimples is particularly thin. As a result, the mid layer can be exposed at the bottom of dimples or seen through. Further, separating, cutting and cracks of the cover immediately below dimples are liable to be generated.
An object of the present invention is to provide golf balls with favorable appearance and durability.